Designing highly delocalized solitons by harnessing the structural parity of π-conjugated polymers

π-Conjugated polymers play a crucial role in modern organic optoelectronics and spintronics. However, a key aspect remains unexplored: how to make a π-conjugated polymer able to host intrinsic solitons just by chemical design without the need of external doping. Exploiting on-surface synthesis and to address such challenge, we present a novel chemical reaction based on the regio- and stereo-selective coupling of indenyl moieties for fabricating π-conjugated acenoindenylidene polymers on Au(111) surface, which feature a longitudinal polyacetylene backbone. We address an aspect of polymer design that was experimentally overlooked, namely the relation between structural parity and electronic properties. We discover that odd-membered polymers exhibit an in-gap soliton state, which, thanks to their low bandgaps, spatially extends several nanometers along the longitudinal polyacetylene backbone. Our findings pave the way for the design of π-conjugated polymers that feature highly delocalized quasiparticles simply through chemical design by exploiting structural parity, while eliminating the need for doping.